The present invention relates to color measurement instruments, and more particularly to such instruments that use polarized light in making measurements.
Optical density is used in the printing arts both as a measurement of ink film thickness and as a rough measurement of color by measuring the amount of light reflected off an object and passing through an optical filter. The amount of light that is reflected by an object is affected by many things, including its surface. An ink with a smooth surface will reflect light differently than one with a rough surface. This difference can have a significant effect when measuring the optical density of a wet ink because, when the ink is first applied, it forms a smooth surface over the top of the rough paper. However, as it dries, it conforms more to the surface of the paper, developing a rougher finish.
Light reflected from a printed surface usually consists of two components. The first component is xe2x80x9cfirst-surfacexe2x80x9d or xe2x80x9cspecularxe2x80x9d reflection, and the second component is back-scattering from below the surface of the ink. The first component is the light reflected from the surface of the ink and is associated with a property known as xe2x80x9cglossxe2x80x9d. The second component is the light reflected from below the surface of the ink. Only the second component depends on the color of the printed ink.
The current method of minimizing the difference between a wet-ink reading and a dry-ink reading is (1) to shine a light through a polarizing filter onto the surface of an object and (2) to detect the light that is reflected off the object through another polarized filter oriented at 90 degrees to the first polarizing filter. The theory behind such an approach is that the light reflected off the surface of the wet ink remains polarized, while the light that passes through the ink and is reflected off the paper becomes nonpolarized. The cross-polarized filter on the sensor side removes the polarized component of the light, leaving only the light that has been reflected off the paper. Therefore, the process more closely simulates the effect of dry ink. For example, when the sample is illuminated at a 45-degree angle, and the reflected light is read at a zero-degree angle (i.e. perpendicular to the sample), the influence of specular reflection is almost completely suppressed.
Unfortunately, polarization weakens the correspondence between the visual impression of the printed sample and the measured density value of the printed ink. When polarization is used, it is important that the spectral and other optical properties of the polarizing filters be taken into account during the design of the instrument. A densitometer that conforms to specifications given in ISO 5-3 and ISO 5-4 without polarization may fail to do so after the addition of polarization filters. Likewise, an instrument with polarization that was designed to conform to specifications given in ISO 5-3 and ISO 5-4 may not conform after the filters have been removed.
Also, there are compromises to the polarization approach. First, the amount of light received by the sensor is greatly reduced by the set of polarizing filters. This increases the difficulty in accurately and repeatably measuring the reflected light. Second, the original characteristics of the light are xe2x80x9clostxe2x80x9d when it passes through the cross-polarized filter. Once the light has been cross-polarized, there is no practical way of determining the component of the light that was removed by the polarizing filters.
Color measurement instruments sold by GretagMacbeth of Regensdorf, Switzerland provide a user the option of taking a polarized reading or a nonpolarized reading. These instruments include polarizing filters that are mechanically moved into or out of alignment with the optical engine. The filters are manually moved in Models D19C and SPM100. The filters are automatically moved in response to keyboard input in Model SpectroEye. In operation, the user selects either the polarized mode or the nonpolarized mode and operates the instrument consistently in the selected mode. If a user were to attempt to obtain both polarized and nonpolarized readings of a single sample, a first reading would need be taken; the filters must be manipulated; and a second reading must be taken. This process would be cumbersome and would often result in sampling of different spots or portions on the object, leading to consistency problems. The two readings also would likely involve different illumination, further contributing to consistency problems.
The aforementioned problems are overcome in a first aspect of the present invention wherein both polarized data and nonpolarized data is acquired in a single reading. More specifically, the reflected light passes through both polarizing and nonpolarizing filters on a single reading. Consequently, polarized and nonpolarized data is acquired from the same sample area under identical illumination. In the preferred embodiment, the filters are mounted in a wheel that rotates during a reading to interpose both polarizing and nonpolarizing filters in the reflected light path.
In a second aspect of the invention, the nonpolarized spectral response is sampled at a first set of wavelengths; the polarized spectral response is sampled at a second set of wavelengths, with the two sets of wavelengths being non-identical; and the spectral responses at non-sampled wavelengths are calculated as a function of the actual nonpolarized readings and the actual polarized readings. This technique enables the presentation of complete spectral information for both polarized and nonpolarized data based on less-than-complete sampled data.
The present invention has several advantages over known color measurement techniques. First, both the polarized data and the nonpolarized data are acquired in a single reading. Second, precisely the same sample spot is measured for both polarized and nonpolarized data. Third, the polarized and nonpolarized data is acquired under identical lighting conditions. Fourth, the user need not reconfigure the instrument between polarized and nonpolarized readings. Fifth, the instrument provides a common hardware platform for a series of instruments with different functionality implemented primarily in software and firmware. Sixth, the instrument is straightforward and produces full spectral data efficiently, economically, and precisely.